Deceased friend

I've found myself in quite a difficult situation which I'll try to articulate without identifying people / places which might be hard, so some detail will need to be vague.
A close friend of mine died a few years ago, she had several adult children and a second husband. All of the children except one live abroad or at the other end of the country. The "child" who lives here has a physical disability and suffers from anxiety and depression to varying degree's.
One of the last discussions I had with my friend was that I would keep in touch with her son and check he was alright.....this was due to the fact that over the year's he's alienated his family and friends totally by his behaviour. I try very hard to empathise with his situation and the difficulties he can experience but he's very manipulative and a deeply unpleasant young man.
I've kept in touch and assisted him as much as I can and tried to encourage him to get involved with groups etc but am hitting a brick wall with everything and recently had to help resolve an issue he'd caused in the local area. I'm now at a loss and feel guilty because I promised my friend I'd keep in touch with him etc but find it harder and harder.
I really would welcome any advice / thoughts on this.